The development of new polymeric materials from renewable resources is gaining considerable attention. Biorenewability is directed toward a sustainable raw material supply where the raw material is renewed from plants or other biological matter. Biorenewable polymers are pursued as environmentally friendly replacements for commodity plastics from petrochemical starting materials. Thermoplastics constitute more than 65% of all global polymer demand and many have the possibility to be recycled by melt-processing. Biorenewable thermoplastics are potentially recyclable, which is advantageous for consumer packaging and other high volume needs.
Although organic polymers from nature have been used for centuries, they are generally not thermoplastic and cannot be processed in the molten state. The first true thermoplastic, celluloid, was developed in the mid-1800s and was created by nitrating cellulose and adding an appropriate softener. A number of other modified natural polymers, such as cellulose acetate and rayon, have enjoyed commercial success. A very recent addition is Plastarch Material (PSM), which is made from modified cornstarch (>80%) and biodegradable additives. Another modern thermoplastic is named ComZein, which is derived from corn protein. These modified natural polymers have various drawbacks and generally do not enjoy the full processing characteristics of petroleum-based synthetic polymers. The goal remains to use low cost readily available starting materials from biorenewable resources to produce thermoplastic polymers that are competitive with current commercial plastics in the marketplace. To date, only two synthetic polymers from renewable feedstocks have exhibited potential to enter the commodity plastics market, polylactic acid (PLA) and poly-β-hydroxyalkanoate (PHA), which is a copolymer of the butanoate and the pentanoate.
Another commercialized polymer is polyglycolic acid (PGA). PGA displays a melting temperature, Tm, of 225-230° C. and glass transition temperature, Tg, of 35-40° C. PGA exhibits a high degree of crystallinity, 45-55%. The solubility of this polyester is somewhat unique. PGA in its high molecular weight form is insoluble and used in relatively small scale, primarily in the medical industry in the form of biodegradable sutures and implantable medical devices. PGA is sold under the trade name Kuredux® for packaging applications because of its excellent barrier properties (100 times better than polyethylene terephthalate). High molecular weight PGA is currently made by chain-growth ring opening polymerization (ROP) of the cyclic dimer, glycolide; however, difficulties persist in the efficient and cost-effective production of glycolide.
Polymers derived from biorenewable C1 feedstocks, in particular, methanol, are particularly interesting in the pursuit of biorenewable polymers. Methanol is commercially produced from methane in natural gas; however, methanol, wood alcohol, was originally synthesized from wood, and can be prepared from other renewable sources, including, agricultural wastes, ruminant emissions, landfill gas, and methane hydrates from the bottom of the ocean. A methanol economy has received serious consideration as a sustainable, bio-based successor to the fossil fuel economy. Polymers derived from this C1 feedstock can be economically viable before and after any demise of the fossil fuel economy.
PGA is an alternating copolymer of the C1 feedstocks carbon monoxide and formaldehyde. The gas phase thermodynamic calculations for the 1:1 incorporation of carbon monoxide and formaldehyde into a polyester indicates only nominally favorable energetics for this process (ΔG=−0.2 kcal/mol at 298 K), and previous copolymerization studies yielded small molecule heterocycles or low molecular weight materials that were often poorly characterized. The copolymerization of formaldehyde, generally provided as paraformaldahyde or trioxane, and carbon monoxide has been disclosed. Cevidalli et al., U.S. Pat. No. 3,673,156, teaches copolymers prepared using a catalyst selected from the group of chlorinated and fluorinated cationic derivatives of group III, IV, V or VIII elements at −110° C. to 250° C., and 30 to 5,000 atmospheres (440 to 73,500 psi) to give copolymers having ester and acetal groups with significant portions of low molecular weight polymer formed. The ratio of the number of ester groups to the sum of the numbers of ester and acetal groups was claimed to range from 0.056 to 0.97 for these copolymers, although no examples exceeded 0.75. Drent et al., U.S. Pat. No. 6,376,723, teaches copolymers prepared using a catalyst selected from acidic compounds with pKa values below −1, and a sulfone solvent where the PGA is suitable as an intermediate for the preparation of ethylene glycol. The PGA is formed at 100 to 10000 kPa (14 to 1450 psi) and 20 to 170° C., although no examples are disclosed of temperatures for the polyaddition reaction in excess of 100° C. throughout the entire reaction. No yields of methylglycolate upon polymer degradation are reported that exceed 94%, with most results yielding much lower than 94%, implying that the proportion of ester groups was much less than 94% from the polymerization. Nelson et al., U.S. Pat. No. 3,383,364 teaches copolymers prepared using a phosphoric acid, a non-protonating Lewis acid, or a free radical generator as catalyst with a reducing agent in solution at 5 to 50,000 psi and −50 to 250° C. to yield a polymer with 1 to 45 mole % CO units. The final polymer displays a “softening” point of 100° C., but no example of an acid catalyzed polymerization at temperatures in excess of 100° C. is disclosed. Modena et al., Journal of Polymer Science Part B: Polymer Letters, 1: 567-570 (1963) teaches the preparation of polymers from CO and formaldehyde using an undisclosed Lewis acid at an undisclosed temperature to yield a copolymer with CO/H2CO ratios of 1/3. Ragazzini et al. Journal of Polymer Science Part A, 2: 5203-12 (1964) teaches the preparation of polymers from CO and formaldehyde using boron trifluoride, as a Lewis acid, at 110° C. to form a copolymer with glycolic ester, diglycolic ester (—C(O)CH2OCH2C(O)—), and acetal units, which upon heating, to 180° C. with additional CO, results in the loss of acetal units but the retention of diglycolic ester units in a polymer having a melting temperature 80° C. below that of PGA. The polymer is soluble in DMF and DMSO.
The high Tm and Tg of PGA limit the highly crystalline polymer's use as a replacement for a commodity plastic. The brown or beige color displayed by PGA also diminishes its use in many packaging applications. To these ends, a PGA or PGA copolymer that is produced primarily from C1 feedstocks to a material with similar properties to PGA prepared by the ring-opening of glycolide or modified in a specifically desired manner to yield an equivalent to another commercial thermoplastic, is a desired goal.